During drilling operations, information is sometimes transmitted to the surface from instruments within the wellbore, and/or from the surface to downhole instruments. For example, signals may be transmitted to or from measuring-while-drilling (MWD) equipment, logging-while-drilling (LWD) equipment, steering equipment, or other equipment. Such information may assist operators in the task of efficiently drilling a wellbore by providing information related to tool-face orientation and formation composition, and allowing commands and configuring of the downhole instruments, among other possible uses.
In some situations, generally after drilling at least a portion of the wellbore, information about the subterranean formation may be acquired using sensors deployed into the wellbore for deep-imaging of the sub-terrain. For example, seismic data may be acquired using geophones, enabling the generation of vertical seismic profiles, and other types of seismic images, to be generated, which may provide insight into the structure, lithology, etc. of the formation. A seismic source, generally a vibrator, is then used to generate seismic waves that propagate though the formation and are detected by the seismic sensors such as geophones, accelerometers or geophones. For borehole seismic imaging, the seismic sensor may cover an adequate extent of the wellbore. In many such applications, the seismic sensors may be moved within the wellbore, while the surface seismic source may be stationary. This type of seismic technique is generally not done simultaneously with drilling operations, but may be done when the drill string is removed, using wireline logging methods associated with surface seismic source.
Another deep-imaging technique may be based on electromagnetic systems. In this technique, an electromagnetic signal is passed through the formation and detected by a receiver. The characteristics of the signal may provide information about the formation within about 50 feet (about 15 m) of the wellbore. Further, in a completed well, cross-well tomography can be performed by electromagnetic system. For electromagnetic tomography, the source and the receiver may be moved to multiple positions to for additional illumination paths. This type of electromagnetic tomography is generally not done simultaneously with drilling operations, but may be done when the drill string is removed, using wireline logging method in one well while the source may be located at the surface or in another well.